High-density polythene plastic is used for purposes that require a greater tensile strength than that found in polyethylenes with less density. Many chemical containers, perishable foods, water containers, fuel tanks and plastic lumbers are made of HDPE. While HDPE has a different chemical composition than other polyethylenes, repairing high density polythene plastic is no different than repairing any other. When heated, polythene melts and the molecules bond with other polythene molecules without changing the chemical composition of the plastic. As a result, HDPE is easily plastic welded.
Assess the damage to the high-density polythene. If the plastic is cracked, measure the length. If it has a hole, measure the diameter. Score a piece of polythene from your scrap piece. Make it two inches longer than the crack and two inches wider, or two inches greater, than the diameter of the hole in the HDPE.
Find the ends of the crack and drill holes in them to assure the crack will not spread after your weld cures. Drill pilot holes every inch on both sides of the crack, 1 3/4-inch from the crack. For a hole, drill pilot holes 1 3/4-inch from the perimeter 1-inch apart. Center the patch you scored from the scrap piece over the crack or hole.
Begin heating the patch by passing the heat gun over it. If you hold the heat gun over the patch for an extended period of time, you will burn it. Hold the heat gun over the patch for a few seconds, then check the patch. When it begins to melt, spread the liquid out towards the pilot holes. You must force the liquid polythene into the holes so the patch will adhere to the damaged plastic. Once you have filled the pilot holes with polythene from the melted patch, pour water on the patch to cure it.